In the MessageBuffer, detect if we've just been called by a fork child.

lib/datadog/statsd/message_buffer.rb

@@ -20,9 +20,20 @@ def initialize(connection,
 
         @buffer = String.new
         @message_count = 0
+
+        # store the pid for which this message buffer has been created
+        update_fork_pid
       end
 
       def add(message)
+        # we are in a new PID, which means the parent process has just forked and
+        # we are currently running in the child: we have to clean the buffer since
+        # we don't want to process/flush the metrics buffered by the parent process.
+        if forked?
+          reset
+          update_fork_pid
+        end
+
         message_size = message.bytesize
 
         return nil unless message_size > 0 # to avoid adding empty messages to the buffer
@@ -42,13 +53,16 @@ def add(message)
         true
       end
 
+      def reset
+        buffer.clear
+        @message_count = 0
+      end
+
       def flush
         return if buffer.empty?
 
         connection.write(buffer)
-
-        buffer.clear
-        @message_count = 0
+        reset
       end
 
       private
@@ -83,6 +97,21 @@ def ensure_sendable!(message_size)
       def bytesize_threshold
         @bytesize_threshold ||= (max_payload_size - PAYLOAD_SIZE_TOLERANCE * max_payload_size).to_i
       end
+
+      # below are "fork management" methods to be able to clean the MessageBuffer
+      # if it detects that it is running in a unknown PID.
+
+      def forked?
+        Process.pid != fork_pid
+      end
+
+      def update_fork_pid
+        @fork_pid = Process.pid
+      end
+
+      def fork_pid
+        @fork_pid ||= Process.pid
+      end
     end
   end
 end